The answer is, in principle, "yes". But this is not purely "unsetting"
an attribute like you call it in the subject. There is another
technology that could be used for such purpose. It is explained in the
Chapter 6 "Function-Operation-Attribute Triples" of the GAP 4
Programming Reference Manual.

Let me also warn you that the approach that once stored value of the
attribute can not be changed later is rather safe to avoid occasional
changes leading to errors. So it is recommended to carefully use this
feature and select attributes for such setup.

For example, Chapter 6 explains how it is used for Sylow p-subgroups.
In this case it is really important to implement key dependent
attribute to store them for different p's. I hope that your case also
actually requires such technique.

Hope this helps. If you have further questions please do not hesitate
to ask me.